Pupils received record GCSE results yesterday with the proportion of top grades - more than one in five GCSEs was awarded an A or A* - rising at the fastest rate in nearly 20 years.

Experts suggested that the rise was linked to a sharp fall in entries as schools refined the subjects candidates were entered for to achieve the five good grades per pupil required to move up the league tables.

The average number of GCSEs taken dropped below eight for the first time in recent years amid suggestions some schools are opting for more vocational qualifications or fewer GCSEs. A growing number of the brightest pupils are also sitting exams early or opting for an elite version, the IGCSE, based on the old O-level.

The exam boards said students were customising the system to take subjects at their own pace. But there were warnings that the trend could splinter the education system with pupils at high-performing schools coming away with qualifications which top universities and employers might value more.

Some of the fastest improvements were in comprehensive schools. The exam boards, represented by the Joint Qualifications Council, presented data which showed that since 2002 comprehensives have improved their proportion of top grades faster than selective and independent schools. The proportion of A grades obtained by independent schools has dropped by 0.3% but this seemed to be an effect of 250 of the highest performing private schools slipping out of the data when they switch to the IGCSE, which was not included in yesterday's results.

The proportion of pupils getting a C or above increased by 2.4 percentage points to 65.7% - the biggest increase since 1990. The proportion getting A* and A went up by 1.2 percentage points to 20.7% - the largest improvement since 1989. The number of entries from the 750,000 candidates decreased by more than 158,000 since last year. Around 48,000 of these were accounted for by a demographic fall in pupil numbers of around 6,000.

Mike Cresswell, the director general of the exam board AQA, said: "More students were taking GCSEs early in November, especially in English and maths, and this meant fewer entries in this summer's results. Students are focusing their efforts more tightly on a smaller range of subjects."

Greg Watson, the head of the exam board OCR, said: "I've spoken to a number of schools who are talking about a more flexible approach to years nine, 10 and 11 about targeting different pupils with different levels of ability to move at different paces in different subjects. It's not uncommon now for some schools to be starting some pupils in some GCSEs in year nine to create more headroom in the last years of school to move on to A-level."

Research by the thinktank Civitas published yesterday suggested that some schools are using vocational qualifications - worth up to four high GCSEs - to boost their overall scores.

Alan Smithers, professor of education at Buckingham University, said comprehensives were being increasingly canny about what subjects they put children in for.

"It's driven by league tables because 31% is happiness to a school and 29% is abject misery," he said. The government has a minimum target of 30% of pupils getting five good GCSEs.

John Dunford, general secretary of the Association of School and College Leaders, said: "Schools are focusing on quality rather than quantity. It makes sense for students to concentrate on eight or nine GCSEs, rather than 11 or 12, and to do them well."

Lord Adonis, the schools minister, said: "I particularly welcome the record results in English and maths and the sharp rise in entries in biology, physics and chemistry. The overall increase in pass rates and top grades is built firmly on improvements in the key subjects of English and maths."

At a glance

· The father of Jimmy Mizen who died in a bakery attack in Lee, south London, days before he was due to sit his GCSEs yesterday spoke of his pride and pain as he collected his son's results. Mizen was awarded C grades in English, English literature, maths, applied science and religious education based on coursework and teachers' assessments. He also received a B grade for his certificate of personal effectiveness and a C grade for adult literacy and numeracy.

· Murdered teenager Ben Kinsella passed his GCSEs with flying colours and would have been able to follow his dream of studying graphics, his family said. Kinsella, 16, died after he was stabbed during a night out in Islington, north London, on June 29. He gained A* grades in art and media; A grades in English language, English literature and religious education; Bs in maths, information and communications technology and single science; and a C in additional science. He was also awarded B grades in adult literacy and numeracy qualifications.

· Three sets of boy twins from Brighton college in East Sussex amassed a total of 48 A* and A graded GCSEs. Adam and Charles Davies, Luke and Miles Holbrook and Dale and Lucas Currell said it was like winning gold in the Olympics.

· An eight-year-old boy was awarded an A* in his GCSE maths examination. Zohaib Ahmed from Chandlers Ford, Hampshire, said he found the test "only slightly hard". His mother Saadi, 36, added: "He started doing simple maths questions and puzzles at two."

· Classmates Barbara Speed and Phoebe Macdonald notched up 21 A* GCSEs between them. Barbara, 16, from Winchester, Hampshire, who achieved 11 A*s, said: "It's like a dream but I did work hard."